1. Field of the Invention
This invention relates to a front and rear wheel turning system for a vehicle in which both the front and rear wheels turn in response to the turning of the steering wheel, and more particularly to such a front and rear wheel turning system which is provided with a power steering mechanism for assisting the driver in turning the steering wheel.
2. Description of the Prior Art
As disclosed in, for instance, U.S. Pat. No. 4,601,357, there has been known a four-wheel steering system in which both the front and rear wheels turn in response to turning of the steering wheel, the rear wheel turning angle ratio, which is the ratio of the rear wheel turning angle to the front wheel turning angle, being changed on the basis of a predetermined rear wheel turning angle ratio characteristic curve. Generally, the rear wheels turn in the direction opposite to the front wheels (the reverse phase) when the vehicle speed is low, which provides a better heading performance, and turn in the same direction as the front wheels (the same phase) when the vehicle speed is high, which improves the running stability of the vehicle. Further, it has been proposed to prepare a plurality of rear wheel turning angle ratio characteristic curves and select one of them according to the running condition of the vehicle. (See U.S. Pat. No. 4,660,844, for instance.)
Some of such four-wheel steering systems are provided with a power steering mechanism which assists the driver in turning the steering wheel.
In the four-wheel steering system having a power steering mechanism, it is preferred that the power steering mechanism be controlled with relation to the rear wheel turning angle ratio characteristic curve since the rear wheel turning angle ratio characteristic curve can affect the steering characteristics of the steering system.